The invention is related to location-determination method and apparatus.
In many applications, it is necessary to estimate the location of objects in their environment. To date, numerous location-determination systems have been proposed for this task. One such system is the global positioning system (GPS). This system includes a number of satellites that orbit the Earth. It also includes GPS receivers, monitoring stations, and differential GPS receivers on Earth.
GPS satellites transmit signals from which GPS receivers can estimate their locations on Earth. A GPS satellite signal typically includes a composition of: (1) carrier signals, (2) pseudorandom noise (PRN) codes, and (3) navigation data. GPS satellites transmit at two carrier frequencies. The first carrier frequency is approximately 1575.42 MHz, while the second is approximately 1227.60 MHz. The second carrier frequency is predominantly used for military applications.
Each satellite uses two PRN codes to modulate the first carrier signal. The first code is a coarse acquisition (C/A) code. This code is a sequence of 1023 elements, called chips, and it modulates at an approximately 1 MHz rate. The second code is a precise (P) code, which repeats on a seven-day cycle and modulates at a 10 MHz rate. Different PRN codes are assigned to different satellites in order to distinguish GPS signals transmitted by different satellites.
The navigation data is superimposed on the first carrier signal and the PRN codes. The navigation data is transmitted as a sequence of five frames. Each frame is six seconds long, and it contains a time stamp for when the frame was transmitted. The navigation-data frames also provide information about the satellite""s clock errors, the satellite""s orbit (i.e., ephemeris data) and other system status data. A GPS satellite receives its ephemeris data from monitoring stations that monitor ephemeris errors in its altitude, position, and speed.
Based on the signals transmitted by the GPS satellites, GPS techniques typically estimate the location of a GPS receiver by using a triangulation method, which typically requires the acquisition and tracking of at least four satellite signals at the 1.57542 GHz frequency. GPS signal acquisition often involves correlation calculations between the received GPS signal and the C/A code of each satellite at various phase offsets and Doppler-shifted frequencies. Code phase and phase delay are other names for phase offset. For each satellite, GPS acquisition techniques record the largest-calculated correlation value as well as the phase offset and Doppler-shifted frequency resulting in this value. GPS acquisition techniques then identify several satellites (e.g., the four satellites) that resulted in the highest-recorded correlation values.
After signal acquisition, a signal tracking process decodes the signals from the identified satellites at the phase offsets and Doppler values associated with the recorded correlation values for these satellites. Specifically, the signal tracking process uses the identified phase offset and Doppler values for the identified satellites to extract each identified satellite""s navigation data. Part of the extracted data is the time stamp information.
From the extracted time data, the signal tracking process can compute the distance between the receiver and the identified satellites. In particular, a satellite""s signal-transmission delay (i.e., the time for a signal to travel from the satellite to the receiver) can be calculated by subtracting the satellite""s transmission time (i.e., the satellite""s extracted time stamp) from the time the receiver received the satellite""s signal. In turn, the distance between the receiver and a satellite can be computed by multiplying the satellite""s signal-transmission delay by the speed of light. The estimated or exact distance between the receiver and a satellite is often referred to as the satellite""s pseudorange.
After signal tracking, a triangulation process typically computes the location of the GPS receiver based on the computed pseudoranges and the locations of the satellites. The location of each satellite identified during signal acquisition can be calculated from the satellite""s ephemeris data. Theoretically, triangulation requires the computation of pseudoranges and locations of only three satellites. However, triangulation methods often use the pseudoranges and locations of four satellites because of inaccuracies in the receiver clock.
Some GPS systems also improve their accuracy by using differential GPS techniques. Such a technique requires the operation of differential GPS receivers at known locations. Unlike regular GPS receivers that use timing signals to calculate their positions, the differential GPS receivers use their known locations to calculate timing errors due to the signal path. These differential GPS receivers determine what the travel time of the GPS signals should be, and compare them with what they actually are. Based on these comparisons, the differential GPS receivers generate xe2x80x9cerror correctionxe2x80x9d factors, which they relay to nearby GPS receivers. The GPS receivers then factor these errors into their calculation of the transmission delay.
Signal tracking has a number of disadvantages. For instance, it is computationally intensive, and hence quite time consuming. Also, signal tracking at worst needs 6 seconds, and on average needs 3 seconds, of GPS data to extract a selected satellite""s time stamp, as the time stamp is embedded in the navigation-data frames that are 6 seconds long. This requirement, in turn, reduces the speed of the location-determination process. Finally, signal tracking and reliable decoding of data require higher signal power than that needed for acquisition. This becomes a significant impediment when the GPS signal is attenuated, as in indoor or urban environments.
Therefore, there is a need for a global positioning system that can quickly identify a GPS receiver""s location. There is also a need for a global positioning system that only needs a small amount of received GPS data to identify the GPS receiver""s location. More generally, there is a need for a location-determination system that addresses some or all of the above-mentioned needs.
Some embodiments of the invention provide a location-determination system that includes several transmitters and at least one receiver. Each transmitter transmits a signal that includes a unique periodically-repeating component, and the receiver receives a reference signal. Based on the received reference signal, the location-determination system identifies an estimated location of the receiver as follows. For each transmitter in a set of transmitters, the system computes a phase offset between the received reference signal and a replica of the transmitter""s periodically-repeating component. The system also identifies an approximate location of the receiver and an approximate receive time for the received signal. The system then uses the identified approximate location and time, and the computed phase offsets, to compute pseudoranges for the set of transmitters. Finally, the system identifies the estimated location of the receiver by using the computed pseudoranges.